<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="form_3.php" method="post">
    <div id="content">
        <input type="text" name="string" value="" />
        <button id="btn">查询</button>
    </div>
</form>

<script>
    document.getElementById("btn").onclick =function () {
        let xmlHttp =new XMLHttpRequest();
        xmlHttp.onreadystatechange=function () {
            if(xmlHttp.readyState ==4 && xmlHttp.status ==200){
                document.getElementById("content").innerHTML=xmlHttp.responseText;
            }
        };
        xmlHttp.open("POST","form_3.php",true);
        xmlHttp.send();
    }
</script>
</body>
</html>